Automatic Focus (AF) function is provided in most of the camera modules equipped in smart phones that are multifunctional mobile telephones, which are highly compatible with general digital cameras, mobile telephones, and the Internet, and which are manufactured on the basis of functionalities of personal computers. Such AF function equipped in such compact cameras often adopts a contrast detection method. The contrast detection method is a method of actually moving the lens, detecting the lens position where the contrast of an object in a picked-up image is maximized, and moving the lens to the position.
The above-described contrast detection method can be realized at a lower cost than that of the active method of irradiating infrared rays or ultrasonic waves onto the object, and measuring a distance to the object by use of the reflected waves. However, the contrast detection method has a drawback that it takes time to search for the lens position where the contrast of the object is maximized. For this reason, there is a demand that after a user presses the shutter button halfway, a process of focusing on the object should be completed as soon as possible.
In the meantime, the number of pixels of the camera modules equipped in the general digital cameras and the mobile telephones is increasing year by year, and even with such compact cameras, high-definition images can be taken. In the high-definition images, defocusing is easily noticeable and more highly precise AF control is demanded.
Additionally, in general, a device that an input signal and a displacement depending on the input signal can be represent by a linear function is referred to as a linear movement device. Such a linear movement device includes, for example, an AF lens of a camera.
FIG. 1 is a configuration view illustrating a control device of a conventional linear movement device disclosed in patent literature 1. The control device of a linear movement device 112 illustrated in FIG. 1 includes a magnetic field sensor 113, a differential amplifier 114, a non-inverted output buffer 115, an inverted output buffer 116, a first output driver 117, and a second output driver 118. The linear movement device 112 is feedback controlled by the control device, and includes a lens (not illustrated) and a magnet 110.
The magnetic field sensor 113 generates a signal based on the detected magnetic field, and outputs an output signal SA. The output signal SA from the magnetic field sensor 113 and a device position instruction signal SB are respectively input into a non-inverting input terminal and an inverting input terminal of the differential amplifier 114. The differential amplifier 114 that has received the output signal SA from the magnetic field sensor 113 and the device position instruction signal SB outputs an operation amount signal SC representing operation amounts (the product of deviation and amplification degree) of the output drivers 117 and 118.
The magnitude of the operation amount signal SC changes the direction and amount of the current flowing across the coil 111 of the linear movement device 112. The current flowing across the coil 111 changes (moves) the position of the linear movement device 112 including the magnet 110. In this situation, the output signal SA from the magnetic field sensor 113 changes in association with the movement of the magnet 110. The control device detects the position of the linear movement device 112 with a change in the output signal SA, and carries out the feedback control so that the detected position corresponds to a position instructed by the device position instruction signal SB.